At the operation of a combustion engine whose fuel system may be supplied with fuels of different quality, it is of interest to be able to determine the quality of the fuel which is used in order to e.g. be able to adapt the engine's operation, as well as the intervals for the performance of various service measures, to the quality of the fuel. One measure of the quality of the fuel is its bulk modulus, i.e. how well the fuel compresses. Prior art provides for determining the bulk modulus of fuels by sending waves through the fuel with an ultrasound sensor and measuring the velocity of sound in the fuel. Such an approach for determining the bulk modulus of the fuel suffers, in addition to the considerable cost of such equipment, from the disadvantage that the fuel is partly degraded by the sound waves, which increases the wear of parts of the fuel system and the combustion engine, which come into contact with the fuel.
For this reason, a number of different methods to determine the bulk modulus of fuels without the use of such a sensor have been proposed. Examples of such are described in WO 2009121652, U.S. Pat. No. 8,215,161, U.S. Pat. No. 6,805,105, U.S. Pat. No. 7,523,723 and EP 1030047 B1. These methods are, however, relatively complicated and/or have some other drawbacks in terms of reliability.